The technical portion of the work of an optician consists in mounting a pair of ophthalmic lenses on a frame that has been selected by the wearer. This mounting comprises two main operations:                centering each lens, which consists in positioning and orienting the lens appropriately in front of the eye of the future wearer, and thus relative to the frame; and then        shaping (edging) each lens by machining or cutting its outline to the desired shape, given the defined centering parameters.        
In the context of the present invention, attention is given to the first operation of centering. Specifically, the optician needs to define the position that the ophthalmic lens is to occupy in the optical frame of reference (typically conventional marks or a centering point), before the lens is shaped to have the final shape it needs to present after shaping, so that the lens will be suitably positioned in front of the corresponding eye of the wearer and will perform the optical function for which it is designed as well as possible.
For this purpose, the optician initially situates the position of the pupil of each eye on the corresponding lens. This serves mainly to determine two parameters that are associated with the morphology of the wearer, namely the pupillary distance or the two half-distances defined as the distances between the pupils of the two eyes and the nose of the wearer, and also the height of the pupils relative to the final outline.
Thereafter, the optician transfers the height as measured in this way onto the non-edged lens, starting from the position of the centering point of the lens as previously identified and that is to be positioned in register with the pupil of the wearer. This transferred height, together with the corresponding pupillary half-distance, then enables the lens to be centered, i.e. positioned appropriately on the frame, and thus enables the final outline to be positioned on the lens.
Nevertheless, it can happen that the final outline protrudes beyond the outline of the non-edged lens, making it impossible to perform a shaping operation to match the intended outline.
In order to remedy this problem, the optician at present acts manually to offset the centering point away from the point that is to come into register with the pupil of the wearer, referred to as the pupil point. This operation enables the entire final outline to be brought within the initial outline of the non-edged lens, however it reduces the optical effectiveness of the lens and it also reduces the visual comfort provided by giving rise to uncontrolled prismatic effects.